The background art of the present invention will be described with reference to a hybrid excavator as an example.
A common hybrid excavator includes an engine, a hydraulic actuator, a hydraulic pump for driving the hydraulic actuator, a generator motor connected to the engine and operable to perform a generator action and a motor action, and an electric device to be charged with electric power generated by the generator action of the generator motor. The generator motor is driven by an electric power stored in the electric storage device to perform the motor action, assisting the engine at a proper time with the power generated by the motor action.
On the other hand, regardless of hybrid excavators or common hydraulic excavators, there is commonly known, as one mode of an engine speed control, a regulation control of changing the engine speed in response to a load torque, as disclosed in Patent Literatures 1 and 2. The regulation control includes adjusting the fuel injection amount so as to change the engine speed between a target speed and a rated speed along a straight regulation line that is set inside an output characteristic line of the engine and slopes downward to the right at a constant angle.
In a hybrid excavator, a load on the engine includes not only a load by the hydraulic pump, but also a load by the generator motor during the power generation. Therefore, the control of the engine speed has to be performed based on a total load torque obtained by adding a load torque of the generator motor to a load torque of the pump. Applying the known technique regarding only a pump load as the engine load directly to a hybrid excavator could not allow the load to be accurately reflected, thus failing to perform basically appropriate control of the engine speed.
Besides, the known technique, where the engine speed is increased with decrease in the load torque, involves an increase in the power for auxiliary devices such as a fan and the like under a low load torque condition, which causes energy loss. Setting the target speed of the regulation characteristic lower, i.e., shifting the regulation line to a lower speed side, in order to reduce the energy loss, permits the increase in a load torque caused by the power generation of the generator motor to further decrease the engine speed in accordance with the regulation characteristic in which the engine speed decreases as the load torque increases. Thus involving an operation under the condition where fuel efficiency of the engine and power generation efficiency of the generator motor are poor makes the energy efficiency be deteriorated.